1) Field of the Invention
The present invention relates to an improvement of a non-aqueous electrolyte secondary cell for the purpose of improving discharging characteristics.
2) Description of the Related Art
In recent years, there has been a rapid reduction in size and weight of mobile information terminals such as mobile phones, notebook personal computers, and PDAs. As the driving power sources for the terminals, non-aqueous electrolyte secondary cells, which have a high energy density and capacity, are widely used.
As a non-aqueous solvent used for the non-aqueous electrolyte of such non-aqueous electrolyte secondary cells, ethylene carbonate (EC) and propylene carbonate (PC), which excel in electrical characteristics, are used.
EC has the advantage of providing a stable covering film with high lithium ion conductivity as a result of reaction with the negative electrode. However, EC is low in oxidative decomposition potential, and thus, is decomposed as a result of reaction with the positive electrode, which generates gas causing the cell to swell. Also, lowness of oxidative decomposition potential poses the problem of a reduction in discharging capacity.
PC, which is high in oxidative decomposition potential, does not pose the problems that EC does. Still, PC forms coating film that is low in lithium ion conductivity as a result of reaction with the negative electrode. This poses the problem of reduced discharging capacity.
Thus, there is a need for a non-aqueous solvent that eliminates the inconveniences encountered with EC and PC.
Japanese Patent Application Publication No. 5-36407 (patent document 1) suggests a technique related to a non-aqueous electrolyte secondary cell.
Patent document 1 uses lithium or a lithium alloy for the negative electrode, and chemical manganese dioxide baked at 440° C. for the active material of the positive electrode. For the electrolytic solution to be immersed in the separator, a solute of lithium trifluoromethane sulfonic acid is dissolved in a mixture of propylene carbonate, ethylene carbonate, 1,3-dioxolane, and dimethoxyethane.
This technique shortens the time for heat treatment for removing moisture out of the positive electrode active material, resulting in a non-aqueous electrolyte secondary cell with sufficient discharging characteristics even during high-load discharging under low temperature.
However, this technique cannot overcome the inconveniences with EC and PC.